Scientists at the Walter Reed Army Institute of Research (WRAIR) and the Walter Reed National Military Medical Center recently published a troubling finding: Escherichia coli carrying a gene conferring resistance to the antibiotic colistin in the urine of a Pennsylvania woman (McGann P et al. Antimicrob Agents Chemother. 2016;60[7]:4420-4421). It was the first time the gene, mcr-1, had been found in a human bacterial infection in the United States.

Mcr-1 thwarts colistin, a 1950s-era antibiotic called out of retirement to treat multidrug-resistant infections including carbapenem-resistant Enterobacteriaceae. Even more concerning, the gene is carried on a plasmid, a short, circular strand of nonchromosomal DNA that can transfer to other types of bacteria, spreading its potentially lethal resistance.